Update heroku deploying docs (declaring java version) #8637
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Purpose
PR updates outdated fragment of heroku deployment documentations. After the java 11 LTS was released, there are not many reasons for using java 9/10(which are end-of-premier-support now) in both new and existing projects, especially since migration to java 11 is trivial. However, if someone needs to use java 9 or 10 for some reasons, the link to heroku documentation can help.